Laser light sources are used in a large number of devices and applications for illuminating objects. In this case, in particular, the advantages of high coherence length and excellent beam profile, i.e. parallelism of the light radiation emitted by the laser light source, the outstanding frequency or wavelength consistency, and the monochromaticity of the light are utilized.
The high spatial and temporal coherence of laser light is due to the stimulated emission by the optical medium in the laser, and has the effect that both the individual wavelets and the waves emitted at different times interfere virtually without limitation. The coherence length l of a laser is related to the linewidth δν by the equationl=c/(2πδν)where c is the speed of light. Accordingly, a linewidth δν=100 MHz corresponds to a coherence length l=0.5 m. The linewidth, or the coherence length, of a laser light source also depends very greatly on the time interval over which the linewidth or the frequency of the laser light source is measured. For example, with large integration times, a laser oscillator exhibits long-term effects which are proportional to the time interval of the measurement and are caused by linear drift of the laser oscillator, for example due to temperature variations.
For some applications, however, a high coherence length is a problem since it can lead to the formation of undesired interference phenomena in the optical beam path. Especially in confocal scanning microscopy, such interference phenomena can induce imaging aberrations.